The present invention relates to a constant voltage generator and more particularly, to a constant voltage generator which is free from variation in a temperature coefficient caused by manufacturing errors, or deviation in manufacturing conditions, of a semiconductor integrated circuit and which is suitable for a semiconductor integrated circuit.
As constant voltage generators, those employing a constant voltage attainable upon forwardly biasing a PN-junction and those employing a breakdown voltage of a PN-junction, that is, employing the so-called Zener diode, are known. In every such constant voltage generator, the attainable constant voltage has temperature dependency. Particularly, those employing a forwardly biased PN-junction have a negative temperature coefficient, and those employing a breakdown voltage have a positive temperature coefficient. Consequently, for the purpose of obtaining a constant voltage free from temperature dependency, it has been proposed to serially connect the above-described respective types of constant voltage generators.
However, since a temperature coefficient has its value varied depending upon an impurity concentration, diffusion depth, etc., even if a series circuit of a forwardly biased PN-junction and a Zener diode have such sizes, depths and impurity concentrations of their respective diffusion regions designed in a manner that the overall temperature coefficient may become zero, the predetermined depths and impurity concentration of the diffusion regions cannot be obtained due to errors in the manufacturing steps or deviation in manufacturing conditions, and therefore, the temperature coefficients of the respective elements are varied, resulting in the disadvantage that a temperature coefficient arises in the overall constant voltage.
Since such variation in a temperature coefficient cannot be corrected once a semiconductor integrated circuit has been manufactured, such integrated circuits are compelled to be rejected. Therefore, there was a disadvantage that a production yield of semiconductor integrated circuits necessitating a constant voltage was low.